United States. Bureau of Labor Statistics. "Persons at Work, by Type of Industry and Hours of Work,"
in
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Employment and Earnings
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https://fraser.stlouisfed.org/scribd/?toc_id=257777&filepath=/files/docs/publications/employment/1970s/1975-1979/empl_031978.pdf&start_page=51 , accessed on February 21, 2018.